1. Field of the Invention
The present invention relates to a semiconductor photodetector, and more particularly to a waveguide semiconductor photodetector to be used for a light receiving module or a light transmitting module in an optical communication system.
2. Description of the Related Art
Research and development to the waveguide semiconductor photodetector have been progressed (or a light receiving module or a light transmitting module in an optical communication system. The waveguide semiconductor photodetector has an optical absorption layer. A light is incident in parallel to a surface of the optical absorption layer. Even if the optical absorption layer is designed to be thin for obtaining high speed performances, then a sufficiently long waveguide length of the optical absorption layer ensures a high photoelectric conversion efficiency. The waveguide semiconductor photodetector may be designed for obtaining both a high speed response due to a shortened carrier traveling time and a high photoelectric conversion efficiency.
One of the conventional waveguide semiconductor photodetector is disclosed in Third Optoelectronics And Communications Conference Technical Digest, July 1998, pp. 354-355. FIG. 1 is a fragmentary cross sectional elevation view illustrative of a conventional waveguide semiconductor photodetector.
An n+xe2x88x92InP cladding layer 102 overlies a semi-insulating InP substrate 101. An n+xe2x88x92InAlGaAs optical guide layer 103 overlies on a selected region of the n+xe2x88x92InP cladding layer 102. The n+xe2x88x92InAlGaAs optical guide layer 103 has a thickness of 0.8 micrometers, and also has a refractive index which is an intermediate level between a core layer and a cladding layer. An i-InGaAs optical absorption layer 104 overlies the n+xe2x88x92InAlGaAs optical guide layer 103. The i-InGaAs optical absorption layer 104 has a thickness of 0.5 micrometers. A p+xe2x88x92InAlGaAs optical guide layer 105 overlies the i-InGaAs optical absorption layer 104. The p+xe2x88x92InAlGaAs optical guide layer 105 has a thickness of 0.1 micrometer. A p+xe2x88x92InP cladding layer 106 overlies the p+xe2x88x92InAlGaAs optical guide layer 105. A p+xe2x88x92InGaAs contact layer 107 overlies the p+xe2x88x92InP cladding layer 106. The lamination of the n+xe2x88x92InAlGaAs optical guide layer 103, the i-InGaAs optical absorption layer 104, the p+xe2x88x92InAlGaAs optical guide layer 105, the p+xe2x88x92InP cladding layer 106 and the p+xe2x88x92InGaAs contact layer 107 forms a waveguide mesa structure.
A silicon nitride film 108 extends over the n+xe2x88x92InP cladding layer 102 and also on side walls and a top of the waveguide mesa structure. The silicon nitride film 108 has a first opening over the p+xe2x88x92InGaAs contact layer 107, and second and third openings over the n+xe2x88x92InP cladding layer 102. N-electrodes 110 are provided in the second and third openings over the n+xe2x88x92InP cladding layer 102. A p-electrode 109 is provided in the first opening over the p+xe2x88x92InGaAs contact layer 107.
The n-electrodes 110 are provided in contact with the n+xe2x88x92InP cladding layer 102 relatively near the waveguide mesa structure so as to reduce a parasitic resistance. The i-InGaAs optical absorption layer 104 is designed to be thin, for example, at 0.5 micrometers in order to obtain a high speed response with a cut-off frequency of not less than 40 GHz at 3 dB-down. The waveguide structure ensures a high photoelectric conversion efficiency at 77%.
The above-described conventional waveguide semiconductor photodetector may reduce, a device resistance. The waveguide mesa structure is directly coated with a thin silicon nitride film, whereby a heat radiation characteristic is poor. An incidence of a high power light into the device is likely to result in an undesirable heat accumulation which may deteriorate or break the waveguide semiconductor photodetector.
In the above circumstances, the development of a novel waveguide semiconductor photodetector free from the above problems is desirable.
Accordingly, it is an object of the present invention to provide a novel waveguide semiconductor photodetector free from the above problems.
It is a further object of the present invention to provide a novel waveguide semiconductor photodetector with a reduced device resistance, a high performance and a high reliability to a heat generation due to an incidence of a high power light without deterioration and break-down.
The present invention provides a semiconductor device including: a waveguide mesa structure including at least an optical absorption layer for photoelectric conversion; and a heat radiation semiconductor layer in contact directly with at least a part of the optical absorption layer for heat radiation from the optical absorption layer, and the heat radiation semiconductor layer being lower in refractive index and larger in energy band gap than the optical absorption layer.